1. Field of the Invention
The invention relates to a wood screw, more particularly to one having a screw thread formed with concave facets.
2. Description of the Related Art
A conventional wood screw includes a shank portion and a head portion on one end of said shank portion. The shank portion generally has a continuous spiral screw thread formed thereon. The screw thread has an upper thread surface, a lower thread surface, and a cutting line that interconnects the upper and the lower thread surfaces at a radial edge of the upper and lower thread surfaces distal to the shank portion. When the conventional wood screw is driven into a wooden board, wood shavings fill the grooves formed between adjacent screw thread sections on the shank portion as the screw thread cuts into the fibers of the wooden board, thereby resulting in a binding force between the screw and the wooden board. However, because the cutting line is in the form of a smooth spiral line, the cutting effect of the screw thread becomes duller as the depth of penetration of the conventional wood screw into the wooden board increases. In addition, the increased resistance can have an adverse affect on the required screw driving force and can result in possible damage to the screw.
It has been proposed heretofore to form the screw thread with densely and successively arranged grooves along the cutting line thereof. While the discontinuous screw thread as such provides additional space for receiving wood shavings as the screw thread cuts into the fibers of a wooden board, a much larger driving resistance is incurred, thereby resulting in a much larger driving force requirement and in possible damage to the screw.
Therefore, the main object of the present invention is to provide a wood screw that has a screw thread formed with concave facets so that the screw can be driven quickly into a wooden board while maintaining a relatively good binding force therewith.
According to the present invention, a wood screw includes a shank portion and a head portion on one end of the shank portion. The shank portion has a continuous spiral screw thread with a plurality of turns formed thereon. The screw thread has an upper thread surface, a lower thread surface, and a cutting line that interconnects the upper and lower thread surfaces at a radial edge of the upper and lower thread surfaces distal to the shank portion. Each of at least some of the turns of the screw thread is formed with from two to six adjacent concave facets along the cutting line. Each of the concave facets has a radial width ranging from about 60 to 120 degrees. Each pair of the adjacent concave facets forms a pointed corner at the cutting line.